The present invention relates to a vehicle steering apparatus, and more particularly, to a steer-by-wire steering apparatus.
As a vehicle steering apparatus for control steered wheels of a vehicle, a steer-by-wire steering apparatus as shown in FIG. 8 is known. In this apparatus, a steering wheel 500 and a steering mechanism 501 connected to steered wheels T (for example, front wheels) are mechanically separated.
In this steering apparatus, the steering wheel 500 and the steering mechanism 501 are not directly connected. A steering angle of the steering wheel 500 is detected, and the steering mechanism 501 is driven by an electric motor 502 in accordance with the detected steering angle. Namely, a rod 510a (steering rod) of the steering mechanism is moved in its axial direction by rotation of the electric motor 502, and the steered wheels T connected to the rod 510a with tie rods and knuckle arms (neither of them are shown) are steered. An elastic member such as a torsion bar (not shown) is coaxially connected to the steering wheel 500 with a steering shaft 503 in between.
A speed reducer 504 having a worm gear combined with a pinion gear, for example, is provided at a lower part at an opposite side of the elastic member from the steering shaft 503, namely, at a secondary side with respect to the elastic member. A reaction force motor 505, which is an electric motor, is connected to the secondary side of the elastic member with the speed reducer 504 in between. The reaction force motor 505 applies a force in the reverse direction (reaction force) from the steering direction to the steering shaft 503 in accordance with the vehicle speed and a road condition, such that an operator feels this reaction force.
In order to steer the steering wheel 500, it is necessary to apply steering torque to resist the reaction torque generated by the reaction force motor 505. A torque sensor 506 is provided at a side of the steering shaft 503 of the above-described elastic member. A detection signal of the torque sensor 506 is outputted to a control circuit 510. A steering wheel angle sensor 507 is provided at the side of the steering shaft 503 with respect to the elastic member, namely, at a primary side of the elastic member, to detect a manipulation amount of the steering wheel 500. The steering wheel angle sensor 507 is closer to the steering wheel 500 than the torque sensor 506 is to the steering wheel 500. A steering wheel angle (turning amount) including the manipulating direction is detected and outputted to a control circuit 510 as a signal expressing a manipulation state of the steering wheel 500 by the steering wheel angle sensor 507.
A rotation angle sensor 509 constituted of a rotary encoder is provided at an output shaft of the electric motor 502 provided at the steering mechanism 501. The rotation angle sensor 509 outputs a rotation angle of the output shaft (not shown) of the electric motor 502, namely, a detection signal indicating a rotation position of the output shaft, to the control circuit 510. The control circuit 510 performs feedback control such as position control to eliminate the deviation between the steered wheel position command based on the steering wheel angle detected by the steering wheel angle sensor 507 and the actual position computed based on the rotation angle detected by the rotation angle sensor 509.
The torque sensor 506 is provided between the steering wheel angle sensor 507 and the reaction force motor 505 in the steer-by-wire steering apparatus for a vehicle constructed as described above. The above-described torque sensor 506 detects torque by detecting a torsion angle of the elastic member such as a torsion bar. Accordingly, if a phase difference occurs between the steering wheel angle sensor 507 and the reaction force motor 505, and thereby a control delay is caused, it has an adverse effect on the control loop of the control circuit 510.
The adverse effect will be explained.
As shown in FIG. 4, the mechanism constituted of the steering wheel 500 and the elastic member 508 such as a torsion bar as described above can be considered to construct a spring vibration system 530.
Specifically, the equation of motion of the spring vibration system 530 is equation (1). Js represents inertia of the steering wheel 500, Ks represents a spring constant of the elastic member 508, Ds represents a viscosity constant of the elastic member 508, Tr represents torque (steering reaction force) generated by the reaction force motor 505, θ1 represents an angle at the primary side of the elastic member 508, and θ2 represents an angle at the secondary side of the elastic member 508.                                           J            s                    ⁢                                                    ⅆ                2                            ⁢                              θ                1                                                    ⅆ                              t                2                                                    =                                            D              s                        ⁢                                          ⅆ                                                                                              ⅆ                t                                      ⁢                          (                                                θ                  2                                -                                  θ                  1                                            )                                +                                    K              s                        ⁡                          (                                                θ                  2                                -                                  θ                  1                                            )                                                          Equation        ⁢                                  ⁢                  (          1          )                    
When the Laplace transformation of the equation (1) is performed and organized, the relationship between θ1 and θ2 is as follows with the Laplacian operator set as s.                                           θ            1                    ⁡                      (            s            )                          =                                                                                                  D                    s                                                        J                    s                                                  ⁢                s                            +                                                K                  s                                                  J                  s                                                                                    s                2                            +                                                                    D                    s                                                        J                    s                                                  ⁢                s                            +                                                K                  s                                                  J                  s                                                              ⁢                                    θ              2                        ⁡                          (              s              )                                                          Equation        ⁢                                  ⁢                  (          2          )                    
The frequency characteristic of equation (2) is as shown in FIGS. 5(a) and 5(b), and θ1 sometimes generates large phase delay with respect to θ2. FIG. 5(a) shows the frequency characteristic of the spring vibration system 530, the vertical axis represents a magnitude, and horizontal axis represents a frequency. FIG. 5(b) shows the frequency characteristic of the spring vibration system 530, the vertical axis represents a phase, and the horizontal axis represents a frequency.
A block diagram of a control system shown in FIG. 8, which includes the spring vibration system 530 (secondary vibration system) as described above is shown in FIG. 6. In FIG. 6, reference character Jh denotes inertia of the reaction force motor 505, and reference character Kr denotes an effect of the steered wheels T (tires) and the road (expressed by the equivalent spring). Reference character G denotes a gear ratio of the steering apparatus, and reference character Gm denotes a gradient coefficient of the reaction force map for weighting.
A steering wheel angle θh detected at the primary side of the elastic member 508 is converted into a steering position command xrd in accordance with the gear ratio G, and steering control is performed by a steering control system 540. In FIG. 6, reference character xr denotes a steered position at which the steered wheels T are positioned by the electric motor 502, or the steering motor. Meanwhile, a steering rod force Fr which the steering rod receives from the road is converted into a steering reaction force command Trd in accordance with the gear ratio G and the reaction force map M, and the steering reaction force Tr is returned to the steering wheel 500 by a reaction force control system 550.
In FIG. 6, the characteristic from the steering reaction force Tr to the steering wheel angle θh is the secondary vibration system explained in FIG. 4, FIG. 5(a) and FIG. 5(b). Therefore, when the operator takes his or her hands off the steering wheel 500, the phase of the steering wheel angle θh (=angle θ1 of the primary side of the elastic member 508) is considerably delayed with respect to the angle θ2 at the secondary side of the elastic member 508 on which the steering reaction force Tr acts, as described above. Due to the phase delay, the closed loop system of the entire steering apparatus can be unstable in the construction in FIG. 6, and vibration occurs to the entire system. The value of the reaction force map M cannot be increased, and therefore the steering apparatus is inferior in the feel of the steering reaction force than ordinary mechanically connected steering apparatus.